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bash:files:read_a_file:read_fields_from_a_file

BASH - Files - Read a file - Read fields from a file

To read fields within each line of the file, additional variables may be used with the read:


Fields are separated with white-space (space or tab characters only)

If an input file has 3 columns separated by white-space (space or tab characters only).

while read -r first_name last_name phone; do
  # Only print the last name (second column).
  printf '%s\n' "$last_name"
done < "$file"

Fields are NOT separated with white-space

If the field delimiters are not whitespace, set the IFS (internal field separator):

# Extract the username and its shell from /etc/passwd:
while IFS=: read -r user pass uid gid gecos home shell; do
  printf '%s: %s\n' "$user" "$shell"
done < /etc/passwd

NOTE: IFS is set to a colon, :, as every field in the passwd file is separated by a colon.


Tab-delimited files

NOTE: For tab-delimited files, use IFS=$'\t'.

WARNING: Multiple tab characters in the input will be considered as one delimiter (and the IFS=$'\t\t' workaround does not work in Bash).


Not knowing how many fields a line contains

You do not necessarily need to know how many fields each line of input contains.

  • If you supply more variables than there are fields, the extra variables will be empty.
  • If you supply fewer, the last variable gets “all the rest” of the fields after the preceding ones are satisfied.

For example:

read -r first last junk <<< 'Bob Smith 123 Main Street Saint Helier Jersey'

NOTE:

  • first: will contain “Bob”
  • last: will contain “Smith”.
  • junk: holds everything else.

Throwaway variable

read -r _ _ first middle last _ <<< "$record"

NOTE: The throwaway variable _ can be used as a “junk variable” to ignore fields.

  • It, and any other variable, can be used more than once in a single read command, if we don't care what goes into it.
  • The first two fields are skipped.
  • The next three fields are reading into variables.
  • The final _ will absorb any remaining fields on the line.
    • It does not need to be repeated there.

WARNING: This usage of _ is only guaranteed to work in Bash.

  • Many other shells use _ for other purposes that will at best cause this to not have the desired effect, and can break the script entirely.
  • It is better to choose a unique variable that isn't used elsewhere in the script, even though _ is a common Bash convention.

bash/files/read_a_file/read_fields_from_a_file.txt · Last modified: 2021/01/26 13:44 by peter

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